In general, The Book Thief's perpetrator, or antagonist, is not just one person. The main group of people who would be considered the antagonists in The Book Thief are the Nazi Party, led by Adolf Hitler. The Nazis are the main reason that characters like Liesel, Max, and Rudy suffer. The Nazis, because of censorship, have instituted the public book-burnings. Before Liesel stole a book from one of these book-burnings, she was just another kid growing up in Nazi Germany. Stealing books gives her a sense of purpose. Also, it is alluded to that the Nazis might be responsible for taking Liesel's mother away, since she may have been a communist. After this, Liesel states that she hates Hitler.
Hitler and the Nazis are the reason Max, who is Jewish, has to go into hiding, not only putting himself at risk but also putting Liesel, Hans, and Rosa at risk if they are caught hiding him. Rudy goes through a lot at the hand of the Nazis, too. Because he is smart and a great athlete, he catches the attention of the Nazis who are in charge of recruiting for a special school for Hitler Youth. His father is taken to the military in his place, since his mother does not want him to leave Himmel Street. Ultimately, Rudy dies in a bombing.
The Nazis are not just one person, and the suffering they caused, although mostly thought about as happening on a grand scale, happened to individuals. The characters in this book, albeit fictional, are just some of those individuals.